The invention relates to wheel speed sensors and in particular to wheel speed sensors mounted in or on a wheel axle.
Sensing devices for wheel rotational speed are common components in vehicle brake anti-skid or anti-lock systems. These systems normally include, in addition to a sensor, a valving mechanism for regulating pressure to the brakes and an electronic monitoring controller which receives signals from the sensor and activates the valving means to vary the brake pressure in accordance with the signals.
The sensors presently in use generally include a rotor, stator, permanent magnet and coil. The rotor and stator are generally provided with a fixed number of teeth or notches. The coil and magnet are arranged relative to the rotor and stator, so that flux lines from the magnet through the stator to the rotor can be interrupted by rotating the rotor relative to the stator. A changing flux field is set up around the coil thereby inducing an alternating current in the coil. The frequency of this current is proportional to the speed of the rotor. Connecting the rotor to the wheel results in an alternating current with frequency proportional to wheel speed.
How and where a sensor is disposed relative to the wheel are important factors affecting cost, design and operation of the sensor. Sensors have been mounted at or near a rotating portion of the brake. In such a position, the sensor is exposed to dirt and extreme temperatures and is not easily accessible for replacement or repair. Mounting the sensors at or near the end of an axle has overcome the access problem but the bearing oil and dirt exposure problem still remains. Also, in cases of most of these "in-axle" sensors, only a portion of the sensor is actually disposed within the axle, with the portion outside the axle serving to obstruct access to the wheel hub, necessitating removal of the sensor before removal of the wheel hub is possible. Furthermore, these in-axle sensors seem prone to axial or rotational movements relative to the axle which affects the signals sent to the controller. Exemplary disclosures of in-axle sensors characterized by one or more of the aforementioned problems are U.S. Pat. Nos. 3,473,120; 3,566,169; 3,915,266 and United Kingdom patent specification No. 1,321,117.